1. Field of the Invention
This invention relates generally to the field of heating, ventilating and air conditioning systems; and relates more particularly to systems and methods for controlling the air flow and the relative pressures within a building.
2. Description of the Prior Art
Variable air volume systems have been widely accepted as the preferred system for building ventilation and typically utilize both a supply fan and return/exhaust fan to provide the ventilating air flow. Control of supply fan volume has been accomplished by using a static pressure sensor remotely located in the air supply system. This sensor is connected to provide control signals to either a vortex damper at the supply fan outlet, motor speed controls, or other means to regulate the supplied air flow, and in that manner the system operates to maintain a preset downstream pressure. Other versions have employed flow sensors to measure what is sometimes referred to as a velocity pressure. These have proved adequate for supply fan control but inadequate when it is desired to control and coordinate the return/exhaust with the supply.
An example of the first mentioned prior system is described in U.S. Pat. No. 4,437,608 wherein static pressure in the supply duct is used to control power to both the drive fan and to the return fan, and by that means it automatically adjusts to system demand. Similarly, in U.S. Pat. No. 4,407,185 a system is described in which the return fan is controlled in response to a static pressure measured at the supply. This technique is used to maintain a negative pressure at the inlet to the supply and thereby draw outside air into the system.
Other recent improvements have involved use of a wide dead band in conjunction with logical pressure sensor controllers to provide control to the inlet dampers (U.S. Pat. No. 4,392,417); while the basic variable volume system has been shown and described in U.S. Pat. No. 4,086,781. In that basic system, a supply duct damper is shown controlled by a pressure sensor located proximate thereto and arranged to maintain constant supply pressure.
Prior art systems, while able to measure static pressure, cannot efficiently control the return/exhaust and coordinate that flow with the supply. This resulting inefficiency is costly, not only in the discomfort caused, but in the higher energy requirements of the system. Moreover, when precise control is required within a building space, such as is often required for laboratory exhaust flow, no effective system has been presented. Lengthy discussions of these problems are provided in the cited prior art.